This invention relates to primary electrochemical cells. More particularly, it is concerned with primary electrochemical cells having an oxidizable active anode material, a cathode current collector, and an electrolytic solution comprising reducible liquid cathode material and an electrolyte solute dissolved therein.
A particularly effective class of primary electrochemical cells which employs soluble or liquid cathode materials, as opposed to more conventional solid cathode cells, has undergone rapid development in recent years. In these cells the active cathode material is usually a fluid solvent for an electrolyte solute which provides conductivity. The active anode of these cells is usually lithium or another highly electropositive metal. During discharge the solvent is electrochemically reduced on a cathode current collector to yield ions, e.g. halide ions, which react with positive metal ions from the anode to form insoluble metal salts, e.g. metal halides. The cathode current collector does not take part in the reaction itself, but simply provides a support on which the reaction can occur, supplying electrons given up during the oxidation of the anode material.
Electrochemical cells of the foregoing type typically produce an output voltage which remains relatively constant to the end of its discharge life when the voltage falls very rapidly. For many applications such end of discharge behavior presents no problems. In certain applications, however, it is desirable that a signal be provided by the cell indicating that it is approaching the end of its useful life. An electrochemical cell employing a cathode current collector having a first catalyst of porous carbon material and a second catalyst or porous nickel material in which the output voltage drops from one level to another level near the end of the discharge life of the cell is described and claimed in the above-mentioned application of Dampier and Schlaikjer.